An electro-explosive device (EED) is an apparatus for initiating the detonation of a primary explosive in a variety of explosive devices such as explosives used in the construction industry, military ordnance, and the inflation charges of automotive air bags. A blasting cap is one example of an EED. In general, an EED receives electrical energy and initiates a mechanical shock wave and/or an exothermic reaction, such as combustion or deflageration, that, in turn, is coupled to an adjacent primary explosive material or pyrotechnic mix to initiate explosion thereof. This explosion can then be coupled to a main charge for initiating explosion of the main charge. The EED has long been used both in commercial and military applications for a variety of purposes such as those mentioned above.
With reference to FIG. 1, a typical prior art EED 10 comprises a thin resistive wire or bridgewire 12 suspended between two posts 14, only one of which is shown. The bridgewire 12 is surrounded by a primary explosive compound or primary charge 18. To initiate combustion of the primary charge 18, a DC or very low frequency current is supplied through lead wires 16 and posts 14 and then through the bridgewire 12. The current passing through the bridgewire 12 results in ohmic heating of the bridgewire 12 and, when the bridgewire 12 reaches the ignition temperature of the primary charge 18, the charge ignites explosively. The explosion of the primary charge then ignites a secondary charge 20, which, in turn, ignites a main charge 22. The typical EED also includes various protective elements, such as a sleeve 23, a plug 24, and a case 26.
Although the EED 10 is a well known device, the electromagnetic environment in which EEDs must operate has changed dramatically over the past four decades. One such change, for example, has been that EEDs are subjected to higher levels of electromagnetic interference (EMI) due to the necessary proximity of EEDs to high power radar and communications equipment, such as on an aircraft carrier flight deck. The EED that initiates an automotive air bag charge may also be subjected to severe EMI during the normal life span of an automobile. Thus, EEDs are today subjected to high levels of EMI in both military and non-military environments.
The presence of intense EMI in the vicinity of EEDs causes a serious problem because the EMI can couple energy either through a direct or indirect path to an EED causing accidental firing. Energy may be coupled directly to an EED, for example, when RF radiation is incident on the EED's chassis wherein the EED acts as the load of a receiving antenna. Alternately, energy may be coupled indirectly to an EED when RF induced arcing occurs in the vicinity of the EED and is coupled to the EED, such as through its leads. Such an RF induced discharge can occur whenever a charge accumulated across an air gap is sufficient to ionize the gas in the gap and sustain an ionized channel.
Another manner in which an EED and its associated explosive may be accidentally discharged is by the coupling of a high voltage electrostatic discharge (ESD) to the EED. Such a discharge, while usually insufficient to heat the bridgewire of the EED, nevertheless can create a sufficiently large electric field between the input pins of the EED to ignite the primary charge. EEDs can also be accidentally discharged by the inadvertent connection of its leads to a voltage supply such as an electrical outlet or the electrodes of an arc welder on a construction site. Such accidents have been known to occur in the past with obviously devastating results.
EEDs that are relatively insensitive to accidental detonation by EMI and ESD have been developed. In some cases, discrete electronic components, including resistors, capacitors, and inductors are connected to the EED to form various types of electrical filters that can block the effects of EMI and ESD. Such filters can usually be classified as L, Pi, or T filters and are well known by those of skill in the art. While these filters can be effective, they have the disadvantage of being relatively bulky, expensive, and requiring substantial space.
Much smaller and lighter EMI and ESD insensitive EEDs have been developed. One such device is disclosed in my pending U.S. patent application Ser. No. 08/518,169. In general, these devices have conductive bridges that are etched or deposited onto a silicon wafer substrate using standard integrated circuit construction techniques. Electronic components, such as diodes and resistive elements, are also formed on the substrate and are coupled to the bridge to form various filters, voltage dividers, shunts, and the like that function to isolate the bridge from the effects of EMI and ESD. The primary charge of explosive material is then packed with high pressure against the device and against the bridge. When a sufficiently high firing current is applied to the device, the material of the bridge explodes in a plasma, which expands outwardly from the substrate and condenses on the particles of the adjacent primary charge. This, in turn, couples energy in the form of heat to the primary charge to ignite it and, in turn, to ignite the explosive device in which the EED is installed.
While these printed circuit type EEDs have proven very successful at providing reliable detonation while being insensitive to accidental discharge by EMI and ESD, they nevertheless also have an inherent shortcoming. Specifically, since the bridge of the device is disposed on the flat surface of the silicon substrate, the plasma explosion of the bridge, when detonated, expands from the surface of the substrate and into the primary charge in a relatively broad and roughly hemispherical pattern. Accordingly, the energy of the exploding plasma dissipates relatively rapidly with distance from the substrate. This can result in the failure to couple sufficient energy to the primary charge to initiate ignition, particularly in instances when the material of the primary charge has migrated away from the bridge as a result of thermal expansion and contraction or mechanical shifting.
To address this problem, a larger volume of conductive material can be incorporated into the bridge to increase the overall energy of the plasma explosion of the bridge, but this carries the disadvantage of increasing the size and hence firing energy of the bridge, which is undesirable for a variety of reasons. Alternatively, a coating of a secondary material, such as zirconium, can be deposited on the bridge to increase the volume of plasma generated when the EED is detonated. While this is effective, it also requires additional manufacturing steps and costs and is still only a somewhat brute force solution to the problem.
Accordingly, there exists a need for an EED that is insensitive to accidental detonation by EMI, ESD, and accidental connection to common voltage sources, that reliably and consistently couples sufficient energy to a primary charge, that is unaffected by migration of the primary or secondary charge of the EED away from the point of plasma explosion, and that is small, lightweight, and economical to manufacture. It is to the provision of such an EED that the present invention is primarily directed.